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  函数的相关知识
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  <time datetime="2021-04-22T06:19:45.000Z" itemprop="datePublished">2021-04-22</time>
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    <a class="article-category-link" href="/categories/%E5%87%BD%E6%95%B0%E5%BC%8F%E7%BC%96%E7%A8%8B/">函数式编程</a>
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  <p>上篇博客大概了解了什么是函数式编程，以及为什么要学习函数式编程。接下来，一起再来了解下函数相关的基础知识。</p>
<span id="more"></span>

<h2 id="一-函数是一等公民"><a href="#一-函数是一等公民" class="headerlink" title="一. 函数是一等公民"></a>一. 函数是一等公民</h2><ol>
<li>函数可作为变量；</li>
<li>函数可作为参数；</li>
<li>函数可作为返回值。</li>
</ol>
<h2 id="二-高阶函数（HOF：-high-order-function）"><a href="#二-高阶函数（HOF：-high-order-function）" class="headerlink" title="二. 高阶函数（HOF： high-order function）"></a>二. 高阶函数（HOF： high-order function）</h2><p>函数可以将用户和运算过程隔离开。用户只需输入参数，即可得到结果，无需关注函数的具体运算过程。用户的输入存在各种类型，为了使函数更具有通用性，一般可以将用户的输入设置为函数。即，使用高阶函数可以使函数变得更具有通用性。满足以下条件之一的函数，即可称为高阶函数：</p>
<ol>
<li>函数作为另一个函数的参数</li>
<li>函数作为另一个函数的返回值</li>
</ol>
<h2 id="三-闭包"><a href="#三-闭包" class="headerlink" title="三. 闭包"></a>三. 闭包</h2><p>闭包是可以在另一个作用域中调用函数 A，并访问函数 A 的内部函数 B，在函数 B 的内部引用到外部函数 A 中的成员变量。</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">A</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="keyword">let</span> msg = <span class="string">&#x27;hello&#x27;</span>;</span><br><span class="line">    <span class="keyword">return</span> <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(msg);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> b = A();</span><br><span class="line">b();</span><br></pre></td></tr></table></figure>

<ol>
<li>闭包的本质</li>
</ol>
<pre><code>函数在执行的时候会被放到执行栈上，执行结束后，会被从执行栈上删除。但函数内部存在堆上的变量由于被外部引用，不得释放。
</code></pre>
<ol start="2">
<li>闭包的意义</li>
</ol>
<pre><code>延长变量的作用域范围。
</code></pre>
<ol start="3">
<li>闭包的影响</li>
</ol>
<pre><code>导致内存泄漏，所以闭包并不会优化程序的性能。
</code></pre>
<h2 id="四-纯函数"><a href="#四-纯函数" class="headerlink" title="四. 纯函数"></a>四. 纯函数</h2><p>相同的输入通过映射始终得到相同的输出，且没有可观察的副作用的函数即可称为纯函数。</p>
<ol>
<li>函数式编程中的函数即为纯函数；</li>
<li>纯函数需要有输入和输出；</li>
<li>举例： <figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> arr = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>];</span><br><span class="line"></span><br><span class="line">arr.slice(<span class="number">0</span>, <span class="number">2</span>);  <span class="comment">// slice 函数即为纯函数</span></span><br><span class="line">arr.splice(<span class="number">0</span>, <span class="number">1</span>);    <span class="comment">// splice 即为不纯函数</span></span><br></pre></td></tr></table></figure></li>
</ol>
<h2 id="五-副作用"><a href="#五-副作用" class="headerlink" title="五. 副作用"></a>五. 副作用</h2><p>副作用让一个函数变得不纯。如果函数依赖外部的状态，就无法保证相同的输出，就会带来副作用。副作用是无法避免的，只能将其控制在一定的可控范围内。</p>
<ol>
<li>副作用的来源</li>
</ol>
<pre><code>配置文件、用户输入、数据库等
</code></pre>
<ol start="2">
<li>副作用的危害</li>
</ol>
<pre><code>降低函数的可扩展性和可复用性
</code></pre>
<h2 id="六-柯里化"><a href="#六-柯里化" class="headerlink" title="六. 柯里化"></a>六. 柯里化</h2><p>当函数有多个参数时，可以将一部分参数（不会轻易改变的参数）传入函数中，在函数内部再返回一个接受剩余参数的函数，在该函数内部返回运算结果。</p>
<ol>
<li>柯里化优点</li>
</ol>
<pre><code>增强函数的复用性、满足单一原则
</code></pre>
<ol>
<li>举例 <figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 应用：将传入的数组转换为字符串</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">transArrToStr</span>(<span class="params">sep</span>) </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params">arr</span>) </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> arr.join(sep);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> joinArr = transArrToStr(<span class="string">&#x27;-&#x27;</span>);</span><br><span class="line"><span class="keyword">let</span> arrStr1 = joinArr([<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]);</span><br><span class="line"><span class="keyword">let</span> arrStr2 = joinArr([<span class="string">&#x27;2&#x27;</span>, <span class="string">&#x27;22&#x27;</span>, <span class="string">&#x27;3&#x27;</span>]);</span><br></pre></td></tr></table></figure></li>
</ol>
<h2 id="七-函数组合"><a href="#七-函数组合" class="headerlink" title="七. 函数组合"></a>七. 函数组合</h2><p>使用纯函数和柯里化容易写出洋葱代码，所以推荐使用函数组合，可以将细粒度的函数组合成新的函数，提供函数的复用性。</p>
<ol>
<li>函数组合默认从右向左执行</li>
<li>函数组合符合结合律</li>
</ol>
<h2 id="八-lodash-库的使用"><a href="#八-lodash-库的使用" class="headerlink" title="八. lodash 库的使用"></a>八. lodash 库的使用</h2><p><code>lodash/fp</code> 模块提供了纯函数的一些操作方法</p>
 
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<script>
  function wait(callback, seconds) {
    var timelag = null;
    timelag = window.setTimeout(callback, seconds);
  }
  !function (e, t, a) {
    var initCopyCode = function(){
      var copyHtml = '';
      copyHtml += '<button class="btn-copy" data-clipboard-snippet="">';
      copyHtml += '<i class="ri-file-copy-2-line"></i><span>COPY</span>';
      copyHtml += '</button>';
      $(".highlight .code pre").before(copyHtml);
      $(".article pre code").before(copyHtml);
      var clipboard = new ClipboardJS('.btn-copy', {
        target: function(trigger) {
          return trigger.nextElementSibling;
        }
      });
      clipboard.on('success', function(e) {
        let $btn = $(e.trigger);
        $btn.addClass('copied');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-checkbox-circle-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPIED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-checkbox-circle-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
      clipboard.on('error', function(e) {
        e.clearSelection();
        let $btn = $(e.trigger);
        $btn.addClass('copy-failed');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-time-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPY FAILED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-time-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
    }
    initCopyCode();
  }(window, document);
</script>
 
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<script>
  if (window.mermaid) {
    mermaid.initialize({ theme: "forest" });
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